Intravenous injection of 0.03 mg of rabbit antibody-nitrogen will passively sensitize a guinea pig so that fatal anaphylaxis will result on injection of 1 mg of antigen in the egg-albumin system or 0.1 mg of specific polysaccharide in the type 3 pneumococcus system. The anaphylactic response is dependent to a very great extent on the amount of antigen used. The best anaphylactic response occurs in the region of large antigen-excess. The isolated guinea-pig uterus has about the same range of sensitivity as the intact animal, though it need contain amounts of antibody of the order of only 0.01 μg for contraction to be produced. The relation of these findings to the in vitro methods of detecting antibody is discussed.